The present invention relates to a projection optical unit and a projection-type image display apparatus using such the unit. In particular, the present invention relates to a rear projection television, video projector, or other projection-type image display apparatuses having a reflection-type image formation device such as a DMD (Digital Micromirror Device) or the like, or a transmission-type image formation device such as a transmission-type liquid crystal device or the like.
Projection optical systems which enlarge and project images formed by an image formation device in a projection-type image display apparatus can be broadly divided into refraction optical systems, mainly comprising lenses and other refractive optical elements, and reflection optical systems, mainly comprising mirrors and other reflective optical elements. In general, since the reflection optical system has no chromatic aberration, it has advantage that finer images can be obtained. In addition, positional relationships between the image formation device and mirrors and positional relationships between the mirrors in the reflection optical system have a much greater effect on optical performances, compared with positional relationships between the image formation device and lenses and positional relationships between lenses in the refraction optical system. In other words, a reflection optical system is sensitive to the positional relationships between the image formation device and the mirrors and to the positional relationships between mirrors.
Japanese Patent Laid-open Publication No. 2004-53658 discloses a projection-type image display apparatus which adopts a reflection optical system as the projection optical system. However, in this projection-type image display apparatus, an image formation device and mirrors which constitute the projection optical system are supported by support structures separated from each other. Consequently it is difficult to maintain the image formation device and the mirrors appropriately in a positional relationship according to an optical design. Specifically, it is difficult to appropriately set and to maintain inclinations and distances of the mirrors with respect to the image formation device. In particular, when a shock acts on the apparatus, shifting tends to occur in the inclinations and distances of the mirrors with respect to the image formation device. Moreover, shifts in the inclinations and distances of the mirrors with respect to the image formation device readily occur due to thermal expansion resulting from temperature changes. Deteriorated optical performance of the projection optical system due to the shifts in inclinations or distances of the mirrors with respect to the image formation device degrades quality of displayed images. Thus, the projection-type image display apparatus in which separate support structures support the image formation device and mirrors lacks reliability with respect to shocks and changes in temperature.